Atomic layer deposition (ALD) is a known process in the semiconductor industry for forming thin films of materials on substrates such as silicon wafers. ALD is a type of vapor deposition wherein a film is built up through self-saturating reactions performed in cycles. The thickness of the film is determined by the number of cycles performed. In an ALD process, gaseous precursors are supplied, alternatingly and repeatedly, to the substrate or wafer to form a thin film of material on the wafer. One reactant adsorbs in a self-limiting process on the wafer. A subsequent reactant pulse reacts with the adsorbed material to form a single molecular layer of the desired material. Decomposition may occur through reaction with an appropriately selected reagent, such as in a ligand exchange or a gettering reaction. In a typical ALD reaction, no more than a molecular monolayer forms per cycle. Thicker films are produced through repeated growth cycles until the target thickness is achieved.
In an ALD process, one or more substrates with at least one surface to be coated and reactants for forming a desired product are introduced into the reactor or deposition chamber. The one or more substrates are typically placed on a wafer support or susceptor. The wafer support is located inside a chamber defined within the reactor. The wafer is heated to a desired temperature above the condensation temperatures of the reactant gases and typically below the thermal decomposition temperatures of the reactant gases. The wafer can sometimes be heated to above the decomposition temperature since some variations on ALD processes (e.g., “cyclical” or “digital” CVD) rely on the decomposition of the precursor onto the wafer surface. In such variants, decomposition can be minimized on the other parts of the reactor by using zonal temperature control (keeping other parts of the reactor cooler than the susceptor) or by decreasing the residence time of the gases in the reactor.
A characteristic feature of ALD is that each reactant is delivered to the substrate in a pulse until a saturated surface condition is reached. As noted above, one reactant typically adsorbs on the substrate surface and a second reactant subsequently reacts with the adsorbed species during the subsequent pulse. To obtain a self-limiting growth, vapor phase reactants are kept separated by purge or other removal steps between sequential reactant pulses. Since growth of the desired material does not occur during the purge step, it can be advantageous to limit the duration of the purge step. A shorter duration purge step can increase the available time for adsorption and reaction of the reactants within the reactor, but the vapor phase reactants are not allowed to mix to avoid the risk of CVD reactions destroying the self-limiting nature of the deposition. As the growth rate is self-limiting, the rate of growth is proportional to the repetition rate of the reaction sequences, rather than to the temperature or flux of reactant as in CVD.
Invariably, deposition occurs on ALD reactor surfaces other than the substrate surface during processing. Over time, a film buildup on the surfaces of the reactor can occur. The film buildup can delaminate from the reactor surfaces and contaminate the substrate surface. Large amounts of loosely adhered film buildup on the reactor surfaces increases the total surface area exposed to a reactant pulse. Hence, this can also increase the pulse and purge time required to saturate the wafer surface
Hot CVD reactant parts similarly face deposition build-up, although such coating can be minimized by keeping reactors parts exposed to reactants gases cooler than the CVD temperature. On the other hand, cooler CVD reactant parts may be subject to reactant condensation or adsorption/desorption, as in ALD reactors, leading to contamination problems again.
Frequent cleaning of the reactor can limit the potential for contamination. However, the reactor is out of service during these cleanings and thus reduces the efficiency of the ALD or CVD process.